In the assembly of some products, it is necessary to inspect the product at a stage in the assembly when the elements to be inspected are hidden from the human eye or a machine vision inspection system. For example, the solder joints attaching some high density integrated circuits to a circuit board can be inspected only after the integrated circuits are in place covering the joints to be inspected.
In such circumstances, an x-ray inspection system may be used. Typically such a system employs an x-ray source, opposing a detector about the part to be inspected. The x-ray source is normally an electron tube accelerating electrons in a vacuum from a cathode to a fixed anode as focussed by one or more focussing grids. The x-ray radiation penetrates any obscuring structure to produce a shadow image or radiograph of the part from which the desired elements may be discerned as variations in x-ray attenuation. For example, a metallic solder joint may be detected as a region of high attenuation contrasting with the uniform and frequently lesser attenuation of the obscuring integrated circuit package and die.
In addition to the usefulness of x-rays in penetrating the obscuring outer structure of a part, x-ray inspection systems are ideally suited to the inspection of extremely small features, such as the bonding wires and an integrated circuit. The short wave-length of x-ray emissions, the ability to produce in the x-ray source an extremely small (0.01 mm) focal spot from which the x-rays emanate, and the simple geometry of an opposed x-ray source and detector permit the ready production of high quality, highly magnified images. The magnification is dependent on the ratio of the separation between the x-ray source and the part, compared to the separation between the x-ray source and the x-ray detector. As the detector is moved further from the point source, provided the part remains stationary, the image size is increased in much the same way as a shadow cast on a distant wall is bigger than a shadow cast on a closer wall. Magnifications of greater than 100:1 are routinely obtained.
The high degree of magnification provided by such x-ray inspection systems requires that the camera, the x-ray source, and the part to be inspected be precisely and stably located with respect to each other. Misalignment by amounts as little as 0.0000492 inches can adversely affect the accuracy of the inspection, particularly where the inspection is performed by a machine vision system which is less accommodating to misalignment error than is a human operator.
The necessary stability and accuracy is normally obtained by rigidly mounting the camera and x-ray source to the frame of the inspection system and separately moving the part to be inspected as precisely located, by registration pins, for example, on a table tied to the same frame by an indexing mechanism.
A rigid mounting of the x-ray source is also undertaken to reduce any physical shock to the x-ray source which may adversely affect the size of the x-ray source's focal spot. Thermal variation are reduced by leaving the x-ray source on at all times. The small focal spot of the x-ray source (less than one-hundredth the diameter of a typical medical x-ray source) is necessary to eliminate a penumbra that may blur a highly magnified image and is accomplished by multiple focussing grids and a special x-ray source anode geometry.
Originally, x-ray inspection systems were used for spot inspections of manufactured parts by a human operator. Nevertheless, with improvements in machine vision techniques, the potential exists for automated inspection of 100% of a production run. Unfortunately, current bulky x-ray inspection systems are not well suited for modern factories where space is at a premium. Also, the need to individually locate each part precisely with respect to the x-ray source and camera, by registration pins or the like, is counter to the high throughput required of a 100% inspection system.